to be added
“Like the preceding specie this curious plump shell is hardly recognisable as an Opisthostoma. The irregular last whorl and the oblique position of the top whorls, however, characterise it as such. It is more cylindrical than O. (P.) turriforme, O. (P.) sciaphilum or O. (P.) senex and has a narrower umbilicus than O. (P.) umbilicatum.” (Benthem-Jutting, 1952)
“Both samples contain shells which are a triffle smaller than the original lot from the type locality Gua Musang in Kelantan, 1939 (Van Benthem-Jutting, 1952, Bull. Raffles Mus. No. 24, p. 51). The measurements of 20 shells taken at random from the lot of Ulu Kenyam Kechil, Ulu Tembeling, Pahang, April 1952 clearly demonstrate this difference. For the rest there are no differences.” (Benthem-Jutting, 1961)
Benthem-Jutting (1952) original descriptions on Opisthostoma (Plectostoma) crassipupa – “Shell pupaefrom to cylindrical, yellowish or fawn. Initial whorls smooth, subsequent ones ornamented by distantly placed fine white ribs, standing out wing-like away from the shell. Between the ribs there is a fine spiral sculpture. Somewhat shining and transparent. Whorls 6-7, rapidly increasing in size, the forst two placed obliquely on the shell axis. Last whorl transversely constricted in the middle, then widening again and somewhat irregularly coiled. Adante to the preceding whorl. Suture deep. Umbilicus narrow, in some specimens hidden by the abnormal curve of the last whorl. Aperture vertical, round. Peristome continuous, almost circular, with a double margin. Operculum unknown.”
(description by van Benthem Jutting, 1952)
The shell is small, fawn yellow to red, shining and somewhat transparent with cylindrical spire and circular in cross-section. Whorls six to seven (tuba is not included), convex with deep suture. The first two whorls are smooth, placed obliquely with respect to the coiling axis. The following whorls ornamented with thin, transverse, and wing-like ribs which and widely spaced (three to four ribs/0.5 mm on the penultimate whorl and five ribs/0.5 mm on the body whorl close to the constriction) crossed by fine spiral striations. Umbilicus is narrow and approximately 0.25 mm across; in some specimens hidden by the tuba. The body whorl is irregular and constricted with transverse palatalis. The tuba is 0.5 whorl long and turn off slightly inwards; attaching the body whorl and projecting a little upward with respect to the coiling axis. Aperture is vertical and round with continuous and double peristome which touches the middle of the previous whorl and clearly turned upward. Operculum is unknown. The shell height is in between 2.4-2.2 mm, the total width is between 1.5-1.3 mm, and aperture diameter is in between 1.0-0.8 mm.
The specimens from Ulu Keniam are distinctly smaller than the specimens collected from Kelantan and Terengganu. Benthem-Jutting (1961) recorded the average height, average width and average height of aperture are 1.86 mm, 1.27 mm, and 0.78 mm respectively.
Opisthostoma (Plectostoma) crassipupa – “Dimensions: Height 2.1-2.4, Breadth 1.3-1.4, Diam. apert. 0.8-1.0 mm.” (Benthem-Jutting, 1952); Opisthostoma (Plectostoma) crassipupa from Ulu Kenyam Kechil – “Dimensions: Height 1.7-2.1, Breadth 1.2-1.3, Diam. apert. 0.7-0.8 mm.” (Benthem-Jutting, 1952)
KELANTAN – “Gua Musang, Kelantan” (Benthem-Jutting/1939); Gua Musang, limestone hill near to the railways station (M.E. Marzuki/2009); TERENGGANU – Taman Negara, Gua Bewah, limestone hill on the Kenyir Lake (T.S. Liew/2010); PAHANG - Ulu Keniam Kecil, Ulu Tembeling, Pahang (Leg. J.A. Hislop/1952); Batu Besar, Ulu Keniam Kecil, Ulu Tembeling, Pahang, (Leg. J.R. Hendrickson/1954)